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Canon XL1S / XL1 Watchdog
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Old June 1st, 2002, 03:32 PM   #1
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625 lines

I know that the XL1 is able to achieve 625 lines television resolution. Since MiniDV tapes can only achieve a maximum of about 525, how does the XL1 achieve the extra 100 lines????

Just a question, which is bugging me,

Hope you can help,

All the best,

Ed Smith
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Old June 1st, 2002, 04:16 PM   #2
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Hi,

The 625 lines in the PAL specification is vertical resolution. It is fixed at 625 by the PAL spec and 525 by the NTSC spec. The lines are fixed and cannot be changed because they are the horizontal lines on your TV or monitor. They are referred to as vertical lines of resolution because they are counted vertically from the top of the screen to the bottom. Horizontal lines of resolution are counted left to right (horizontally on your set) and run top to bottom. Horizontal lines of resolution are variable and the vertical lines are not.

Horizontal resolution can be measured at different places. The lens, chip, camera head, tape and various output points (including the monitor). The resolution can be measured optically and electronically (bandwidth that the signal occupies). That is why the lenses are rated at over 600 lines. The tape and the tape/head speed are probably the limiting factor in terms of horizontal resolution.

Jeff
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Old June 2nd, 2002, 10:13 AM   #3
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For DV, horizontal video resolution is limited by the DV specification, a 720 pixel line tha scales to 540 pixels less after overscan on typicl TV sets, resolution speaking. However, actual realized useful resolution will be even less (Kell? effect).
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Old June 2nd, 2002, 10:50 AM   #4
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~So then for the XL1, what are the factors which are giving it the extra 100 lines?

Is it the lens, tape heads etc giving it more horizontal lines?

Say it is the lens on the camera, if I were to use the 1.6x telephoto adapter; I would be loosing more lines?

All the best,

Ed Smith
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Old June 2nd, 2002, 02:20 PM   #5
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The XL1's and almost all camcorders are limited by electrical and mechanical components. Usually the optical components are not the weakest links in the chain. The lenses and camera head specs. will usually be higher than the recording section. Better lenses will have fewer aberrations and other defects (astigmatism, coma, curvature of field, longitudinal chromatic aberration, lateral chromatic aberration, spherical aberration, distortion, and flare). Because their aberrations and defects are better corrected they will have better resolution, definition and acutance (combination of sharpness and contrast).

So now you have this great optical image and here is where it starts to fall apart. Every component will start to degrade the image quality. First the optical block (prism) the 3 CCDs are in, then the CCDs themselves etc. I think you get the picture. The picture quality really starts to fall apart when you go to tape. Hence, the difference between the lines of resolution the camera head creates (600+ lines) and the recorder section lays to tape (around 500 lines).

Could the MFG. improve tapes, tape/head speed etc. to exceede 500+ lines? Yes, but it would exceede the specifications of mini DV and you would have compatability problems with various playback and display devices. Does this mean all images look the same? No, obviously not. Some images and camcorders produce better images than others. How? Mostly by better optical performance and more precise electrical and mechanical components. Tape moves more precisely, heads spin more precisely etc. Sloppy tape movement causes the error correction to kick in and degrade the picture.

But it all starts with light passing through a few glass elements and striking the surface of the CCDs. Everything else being equal the better glass produces better images.

Jeff
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Old June 2nd, 2002, 08:47 PM   #6
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The XL1 achieves about 500 video lines of resolution at the analog output, measured with a standard resolution chart. That is the practical limit of the CCDs using pixel shift. The lens is capable of 600 to 1000 lines depending on setings and what part of the imae you are considering. What is not published (that I have seen anyway) is the MTF for the systems which is more important for sharp, snappy images.

FWIW: When designing mass produced consumer products the individual components are usually matched quality wise so that no one is significantly better than the others in the same device. To do otherwise would be to increase cost to no significant benefit.
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Old June 4th, 2002, 12:11 PM   #7
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Cheers

Thankyou very much for the replies it put that into perspective.

All the best,

Ed Smith
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